2. H - H
is a pure covalent bond
The electrons are shared equally between the 2
atoms – they have the same electronegativity
Electronegativity - is a measure of the
attractiveness of an atom for electrons in a
shared pair (covalent bond)
molecule is non-polar
the centre of positive and negative charges are
in the same place
3. H - Cl
electronegativities
2.1 3.0
Cl attracts shared electrons more than H
therefore it is slightly negative and H is
therefore slightly positive
Difference is 0.9
+
H - Cl
-
this is a polar covalent bond
and the molecule is also polar
4.
5. To show if a compound [usually a
liquid] is polar
Pour it from a burette past a
charged rod
[a pen rubbed vigorously on a
woollen pullover is an easy way of
doing this.]
6. Non-Polar
molecules
• If the liquid is not
polar it will pass the
rod un-deflected
• The stream will be
deflected if it is polar
• If the charge of the
rod is reversed the
stream still bends –
opposite end of
molecule is attracted
Charged
Rod
Polar
molecules
7. Are all molecules with
polar bonds polar?
Let us look at a few examples and see.
8. +
H— O — H
Centre of
positive
charge
-
+
Centre of
negative
charge
Centre of
-ve
charge
+ H
-
O
H+
Centre of
+ve
charge
H2O – What shape is it?
Could be linear
or angular
If linear it will be non polar because the centre of + and –
would be in the O atom
If it is angular
As electron pair repulsion theory suggests
it will be polar
It is deflected by a charged rod so it is polar
9. δ+
δ+
δ-
Cl - Be - Cl
electronegativities
3.0
1.5
3.0
Cl is δ- Be is δ+
Bonds are polar covalent
Centre of + charge is in Be
Centre of -ve charge is half way between Cl’s i.e.in the Be
atom
Charges are not separated therefore the molecule is nonpolar
It would flow un-deflected past a charged rod
10. Other molecular shapes
NH3 is pyramidal in shape and is polar and because the
centres of positive and negative charges are not in the same
place
N
H
Centre of
positive
charge
N
Centre of
negative
charge
H
Centre of
positive
charge
F
B
F
F
Centre of
negative
charge
While BF3 is planar and therefore non-polar because the
centre of positive and negative charges are in the same place
11. Carbon dioxide is non-polar. What shape is it?
O=C=O
Centre of
positive
charge
Centre of
negative
charge
C
Centre of
positive
charge
Centre of
negative
charge
It is linear because this is the only conformation that can put
the centres of positive and negative charges in the same
place.
We will deal with this topic [molecular shapes] in more detail
in another lesson
12. If a polar molecule is placed in a
charged field it will turn
+
H - Cl -
Before switched on
20. Type of bond
[Pure] Covalent
Electronegativities differ by less than 0.4
Polar Covalent
Electronegativity difference greater than
0.4 but less than 1.7
Ionic
Electronegativity difference greater than
1.7
